1. Field of the Invention
The present invention relates to a mobile communications system, and more particularly to a method for controlling a reference value for used in the closed loop control performed between a base station and a mobile station.
2. Description of Related Art
In a code division multiple access (hereinafter referred to as CDMA) mobile communications system, a service area is composed of a plurality of areas. The CDMA mobile communications system performs a two-way radio communication by means of CDMA transmission between a base transceiver station (hereinafter referred to as BTS) disposed at each area and a mobile station (hereinafter referred to as MS) located in an area.
In the CDMA transmission, a transmitting station performs modulation of a transmission signal before spreading the signal band to a wide band using a spreading code that is one of spreading codes that are perpendicular to each other and are previously assigned to every MS, and then transmission is performed. On the other hand, at a receiving station, the received wide-band signal is despread to an original narrow-band signal using the same spreading code before the narrow-band signal is demodulated. Thus, since the spreading codes allow identification of individual communication channels, the same frequency bands are used for all the channels in the CDMA mobile communications system. Further, in the CDMA mobile communications system, a plurality of BTSs can be connected to the MS at the same time. This causes smooth area switching in area boundaries by hand over processing, which is one of the advantages obtained in the CDNA mobile communications system.
On the other hand, in such a CDMA mobile communications system using the same frequency band and different spreading codes, a signal generated by a spreading code becomes a cause of interference noise for a signal generated by another spreading code. Thus, in order to realize the high sensitivity between the BTS and the MS while keeping the interference with other signals at the minimum level, the mutual transmission power control is employed using the signal to interference ratio (hereinafter referred to as SIR), which is defined as a ratio of the receiving power of a desired signal to the receiving power of another signal.
FIG. 1A shows an example of schematic configuration of a CDMA mobile communications system. The service area of this CDMA mobile communications system includes a plurality of areas, in which N base transceiver stations BTSs 101 to 10N are located, respectively, and are connected to a host station, which is a radio network controller (hereinafter referred to as RNC) 11 through network channels. Further, a mobile station (MS) 12 performs radio communication with a plurality of BTSs that are located in the network area by means of CDMA transmission. The MS 12 may be connected to a plurality of BTSs depending on its location and performs radio communication with the BTS having the highest sensitivity at that time. Such a state is defined as a diversity handover (hereinafter referred to as DHO) state.
The MS 12 performs the high-speed closed loop control with the BTS in charge for the purpose of transmission power control. In the description below, it is assumed that the MS 12 is connected to BTSs 101 and 10N. The high-speed closed loop control is a transmission power control performed by an instruction of increase or decrease of transmission power to the opposed station between the MS 12 and each of the BTSs 101 and 10N when transmitting an uplink frame signal from the MS 12 to the BTSs 101 and 10N and when transmitting downlink frame signals from the BTSs 101 and 10N to the MS 12.
FIG. 1B shows a frame format of a signal transmitted between the MS and the BTSs. A frame signal 11 which is applicable to both uplink and downlink frame signals is composed of a plurality of slots 131 to 13N, A transmitter power control (hereinafter referred to as TPC) signal 15 is positioned at the specific location of a predetermined slot 13, among the M slots. The TPC signal 15 is composed of, for example, 2 bits. When the TPC signal is xe2x80x9c11xe2x80x9d, it indicates an instruction of increase in the transmission power and, when the signal is xe2x80x9c00xe2x80x9d, it indicates an instruction of decrease in the transmission power.
Such a TPC signal is generated in a mobile station side or abase transceiver station side. Here in this case, it is assumed that the TPC signal is generated in BTS side. Such a BTS receives an uplink channel frame signal from the MS and compares the receiving SIR of this uplink signal with a predetermined referents SIR (hereinafter referred to as Sref) to perform the transmission power control far the MS. The reference SIR Brat is a predetermined reference value fox attaining a desired frame error rate allocated to every BTS. Hereafter, a frame error rate is referred to as FER.
More specifically, the BTS monitors an uplink frame signal received from a MB and, when receiving an uplink frame signal, measures SIR of the received uplink frame signal. If the measured SIR is not smaller than the reference SIR Sref, then it is determined that the received signal level is high enough to satisfy the desired FER. However, in the case where the transmission power of the MS is too great, it may cause interference to communications of other MSs located in the same area, resulting in deteriorated communication quality as a whole. Therefore, in the case of sufficiently high received signal level, the BTS generates the TPC signal instructing the MS to decrease in transmission power and adds it to the downlink frame signal. On the other hand, if the measured SIR is smaller than the reference SIR Sref, then the BTS generates the TPC signal instructing the MS to increase in transmission power and adds it to the downlink frame signal. When receiving the TPC signal from the MTS, the MS decreases or increases its transmission power depending on the received TPC bits.
Thus, in the CDMA mobile communications system, the transmission power of the MS is controlled from the opposite station (here, BTS) by adding the TPC signal generated based on the uplink frame signal to the downlink frame signal between the BTSs and MS. Such a control is called as a high-speed closed loop control.
Regarding the high-speed closed loop control, several controllers and control methods have been proposed. For example, xe2x80x9cMobile station transmission power controllerxe2x80x9d In Japanese Patent Unexamined Publication No. 6-132782 discloses a technique of maintaining communication quality depending on the distribution of mobile stations. More Specifically, BTS calculates an evaluated SIR value which is the difference between a local station SIR value and an average SIR value of overall service areas, and then compares the local station SIR value with the upper limit and lower limit threshold values, whereby a transmission power required to the opposite party is set to a value proportional to the SIR evaluated value based on the comparison result.
Further, xe2x80x9cTransmission Power Control Methodxe2x80x9d in Japanese Patent Unexamined Publication No. 6-276130 discloses a technique in which d transmission power from the opposite party is set to a range which does not exceed predetermined upper limit value and lower limit value reflecting the updated amount xcex94P which is the difference between the target SIR value and a local SIR value at the present time and the transmission power is controlled so that SIR values of the adjacent BTSs become the same level.
Further, xe2x80x9cTransmission Power Control Method and Transmission Power Control Apparatusxe2x80x9d in Japanese Patent Unexamined Publication No. 8-32514 discloses a technique in which a transmission power control is carried out by the above-mentioned high-speed closed loop control which can be usually performed with high accuracy and, when a change of surrounding propagation situation occurs such that a received power is rapidly increased, the optimized transmission power can be obtained for a short time by an open-loop control.
In the case of the DHO (diversity hand-over) state where the MS 12 is simultaneously connected to the plurality of BTSs 101 through 10N as shown in FIG. 1A, the TIC signal of the high-speed closed loop control is independently received from each of the plurality of BTSs. However, the MS 12 gives priority to the instruction of decrease in the transmission power and determines the transmission power for the uplink channel. Thus, the reference SIR Sref of each BTS which determines the transmission power of the MS is set to the minimum value which keeps interference with other channels at the minimum level and satisfies the desired communication quality.
However, a relationship between the required communication quality and the SIR varies due to fluctuated propagation properties resulting from a change of the moving speed of the MS itself and a change of the number of connected blanches and the like. When a predetermined reference SIR Sref is used, the required communication quality may not be satisfied or excess communication quality may be given. Accordingly, to correct the reference SIR Sref, an outer loop control is carried out.
The outer loop control is described in xe2x80x9cSpecifications of Air-interface for 3G Mobile System Ver 1.0xe2x80x9d(Association of Radio Industries and Businesses: ARIB). The outer loop control of an uplink channel described in this paper causes the reference SIR Sref to be updated based on quality information at the BTS in order to satisfy the required communication quality focusing on the average FIR and the average bit error rate (BER). Although the updating algorithm of the outer loop control is applied to the downlink channel in the above-mentioned paper, the updating algorithm can be also applied to the uplink channel. That is, using the result of cyclic redundancy check (hereinafter referred to as CRC) of a frame signal in the uplink channel from the MS as quality information, the updating of the Sref is carried out based on this result.
FIG. 2A shows the outline of a sequence between an MS and a BTS in which the above-mentioned high-speed closed loop control and the outer loop control are performed. When a frame signal 25 on an uplink channel from the MS is first received at the BTS, the BTS measures an SIR of this uplink frame signal 25 (step 26). Further the BTS checks CRC result of the received uplink frame signal 25 and autonomously performs update of the reference SIR Sref at the BTS in accordance with the CRC result (step 27). In the case where the CRC result exhibits xe2x80x9cNGxe2x80x9d which indicates occurrence of error bits, the receiving level is often insufficient and therefore the reference SIR Sref is increased to cause the MS to increase in transmission power according to the high-speed closed loop control.
On the other hand, in the case where the CRC result exhibits xe2x80x9cOKxe2x80x9d which indicates no occurrence of error bit, the receiving level is often sufficient and therefore the reference SIR Sref is decreased to cause the MS to decrease in the transmission power by the high-speed closed loop control. By comparing the thus updated Sref by the outer loop control with the measured received SIR, the TPC signal is generated and transmitted to the MS using a downlink frame signal 28. By looking at the TPC signal contained in the received frame signals in the downlink channel, the MS increases or decreases in transmission power control.
FIG. 2B shows the outline of a control flow of Sref update processing in a BTS. The BTS monitors a received signal in frames on the uplink channel from the BS (Step S30). When the BTS receives one frame (YES at Step S30), CRC check is performed using the CRC bits contained at a predetermined location of the frame signal (Step S31). When no error occurs, that is, when the CRC result is xe2x80x9cOKxe2x80x9d (YES at Step S32), a SIRdec [dB] which is a predetermined decrease value is subtracted from the reference SIR Sref so that the Sref is updated to a decreased value (Step 33). On the other hand, when the CRC result is xe2x80x9cNGxe2x80x9d (NO at Step S32), a SIRinc [dB] which is a predetermined increase value is added to the reference SIR Sref so that the Sref is updated to an increased value (Step 34). As described above, when the CRC result is xe2x80x9cOKxe2x80x9d, the bit error does not occur and the Sref is decreased so that the transmission power of the MS is not excessively set. And when the CRC result is xe2x80x9cNGxe2x80x9d, the bit error occurs and the Sref is increased so that the transmission power of the MS is increased. Thus, a receiving level which satisfies the target communication quality can be obtained.
The above-described SIRdec and SIRinc have a relationship shown in the following equation:
SIRdec=SIRincxc3x97FERtg/(1xe2x88x92FERtg) . . . (1),
where FERtg is the required quality of FER.
FIG. 3 shows a time-varying reference SIR Sref in the case of the outer loop control on condition that the FERtg is set to 10% in the above equation (1). This means that an addition is performed by an amount of SIRinc 35 at a ratio of 10% and a subtraction by an amount of SIRdec 36 is performed at a ratio of the remaining 90%. In other words, there is a relationship such that when an addition is once performed by the SIRinc, the Sref is gradually decreased to the original low Sref by the subtraction with SIRdec at the remaining nine times. In the case where the SIRdec has the same value as the SIRinc regardless of such FERtg, when the FERtg is 10%, the Sref significantly becomes small by the subtraction of SIRdec. In general, the FERtg is 1% or less and the SIRdec becomes smaller than the SIRinc.
Regarding to such an outer loop control, xe2x80x9cTransmission Power Control System for Mobile communications systemxe2x80x9d in Japanese Patent No. 283034 discloses a technique in which the number of times a received signal quality has been deteriorated is measured and when the deterioration count exceeds a specified value, the Sref is updated at regular steps.
As described above, in the CDMA communications system to which the above conventional outer loop control was applied, in the case of the DSO state where the MS and the plurality of BTSs are connected at the same time, an update operation of the Sref is performed by using the CRC results of uplink frame signals independently received by every BTS as described above. As a result, there is developed a problem such that variations in Sref occur among the plurality of BTSs connected to the MS in the DHO state. That is, the MS in the DHO state decreases in transmission power depending on an instruction received from a BTS having the most excellent propagation condition according to the high-speed closed loop control as disclosed in Japanese Patent Unexamined Publication No. 6-132872, 6-276130 or 6-32514. Therefore, there is a high possibility that a receiving SIR does not reach the reference SIR Sref in the BTS which is not involved in this high-speed closed loop control (hereinafter referred to as BTS_n). In such a case, the BTS_n is controlled so that the Sref is increased by the outer loop control. In general, a plurality of BTSS connected to the Ms, which is in the DHO state, have large differences in propagation conditions, respectively. Accordingly. there is a high possibility that a specific BTS alone can be kept contributing to the high-speed closed loop control for a long time. Therefore, at the BTS_n, an update operation by the increase of the Sref is repeated, and the Sref is rapidly increased. As a result, when a comparison of the BTS which contributed to the high-speed closed loop control and the BTS_n which did not contribute thereto is made, large differences between the respective Srefs occur.
If the MS approaches the BTS_n which is in a state where the Sref is excessively increased, then the Sref is first decreased by such an outer loop control disclosed in, for example. Japanese Patent No. 2823034. However, since the SIRdec is smaller than the SIRinc as described above, a long time is needed until the Sref, which was excessively increased, is decreased to an appropriate value. Since the BTS_n requires an excess transmission power from the MS for the long time, although the BTS_n is a BTS having the most excellent propagation condition, the high-speed closed loop control contributes to another BTS. Accordingly, frame signals are transmitted from the MS with an excess transmission power. This excess transmission power increases the interference power for the surrounding other MSs, resulting in deterioration of transmission quality in an overall service area. Further, in the case where the BTS connected to the MS is only the BTS_n in a state that the Sref is excessively increased, the high-speed closed control is performed with the excess Sref of the BTS_n, whereby the interference power for the surrounding other MSs is also increased and deterioration of the transmission quality in an overall service area occurs.
The above-described disadvantages are caused by the fact that it cannot be determined which of the BTSs does not contribute to the high-speed closed loop control in the DHO state. As a result, the transmission quality in an overall service area is deteriorated.
Accordingly, an object of the present invention is to provide a CDNA mobile communications system which can determine which of the base transceiver stations BTSs does not contribute to the high-speed closed loop control in the DHO state.
Another object of the present invention is to provide a CDMA mobile communications system allowing the deterioration of communication quality to be suppressed even though variations in reference SIR Sref occur from BTS to BTS.
Still another object of the present invention is to provide a CDMA mobile communications system that can minimize variations in reference SIR Sref from BTS to BTS to achieve the stable quality of communication,
According to the present invention, it can be determined which of BTSs does not contribute to a high-speed closed loop control in the DHO state. Further, an appropriate reference SIR Sref can be set for a short period even if there is a wide range of variations in Srefs among BTSs, whereby deterioration of the communication quality due to an excess transmission power of a mobile station can be decreased. Further, the difference among Srefs of BTSs can be minimized to keep the communication quality.
According to a first aspect of the present invention, a method for controlling a first reference SIR (Signal-to-Interference power Ratio) which is used as a reference of a closed loop control by a first radio station to control transmission power of a second radio station connected to the first radio station by radio in a mobile communications system, includes the steps of: a) determining whether a first radio channel used to communicate with the second radio station has sufficient communication quality; b) when the first radio channel has insufficient communication quality. Increasing the first reference SIR by a predetermined increase amount; c) when the first radio channel has sufficient communication quality, determining whether the first radio station is involved in the closed loop control controlling transmission power of the second radio station; d) when the first radio station is involved in the closed loop control, decreasing the first reference SIR by a predetermined first decrease amount; and e) when the first radio station is not involved in the closed loop control, decreasing the first reference SIR by a predetermined second decrease amount which is greater than the predetermined first decrease amount.
The step (c) preferably includes the steps of: c.1) detecting an increase amount of the first reference SIR over a past time period from a current time point within a predetermined time period; c.2) determining whether the increase amount of the first reference SIR is smaller than a predetermined thresholds c.3) when the increase amount of the first reference SIR is smaller than the predetermined threshold, determining that the first radio station is involved in the closed loop control; and c.4) when the increase amount of the first reference SIR is not smaller than the predetermined threshold, determining that the first radio station is not involved in the closed loop control.
Further, the step (e) may includes the steps of: counting the number of times the reference SIR is increased at the step (a) to produce an increase count; when the increase count for the predetermined time period is smaller than a predetermined count threshold, determining that the first radio station is involved in the closed loop control; and when the increase count for the predetermined time period is not smaller than a predetermined count threshold, performing the step (c.1)
According to a second aspect of the present invention, at the Second radio station, the following steps are preferably performed: f) determining whether a second radio channel used to communicate with the first radio station has sufficient communication quality: g) when the second radio channel has insufficient communication quality, increasing a second reference SIR by a predetermined increase amount, wherein the second reference SIR is used as a reference of the closed loop control by the second radio station to control transmission power of the first radio station; h) when the radio channel has sufficient communication quality, decreasing the second reference SIR by a predetermined decrease amount: and i) when the second reference SIR has been increased at the stay (g), sending a reference increase notification to the first radio station. In this case, the step (c) preferably includes the steps of: c.1) counting the number of times the reference increase notification has been received from the second radio station for a predetermined time period, to produce a notification count) c.2) when the notification count is not smaller than a predetermined count threshold, determining that the first radio station is involved in the closed loop control; and c.3) when the notification count is smaller than the predetermined count threshold, determining that the first radio station is not involved in the closed loop control.
According to a third aspect of the present invention, a method includes the steps of: a) determining whether a radio channel used to communicate with the second radio station has sufficient communication quality; b) when the first radio channel has sufficient communication quality, decreasing the reference SIR by a predetermined decrease amount: c) when the first radio channel has insufficient communication quality, determining whether the first radio station is involved in the closed loop control controlling transmission power of the second radio station: d) when the first radio station is involved in the closed loop control, increasing the reference SIR by a predetermined increase amount; and e) when the first radio station is not involved in the closed loop control, causing the reference SIR not to be changed.
The step (c) may include the steps of: c.1) calculating an average reference mover a predetermined past time period from a current time point; c.2) calculating an average receiving SIR over the predetermined past time period by averaging SIRS of signals received for the predetermined past time period; c.3) determining whether a SIR difference between the average reference SIR and the average receiving SIR is smaller than a predetermined threshold; c.1) when the SIR difference is smaller than the predetermined threshold determining that the first radio station is involved in the closed loop control; and e.3) when the SIR difference is not smaller than the predetermined threshold, determining that the first radio station is not involved in the closed loop control,
According to a fourth aspect of the present invention, in a mobile communications system composed of a plurality of base transceiver stations and a plurality of mobile stations, a method for controlling a first reference SIR (Signal-to-Interference power Ratio) which is used as a reference of a closed loop control by a base transceiver station to control transmission power of a mobile station connected to a plurality of bass transceiver stations, includes the steps of:
at the mobile station, a) measuring a second receiving SIR by receiving a broadcasting signal from each of the base transceiver stations connected to the mobile station; b) finding a first bass transceiver station having a maximum second receiving SIR among the base transceiver stations connected to the mobile station; c) selecting a second base transceiver station such that a SIR difference between the maximum second receiving SIR and a second receiving SIR of the second base transceiver station is not smaller than a predetermined SIR difference threshold; d) sending a notification including an identification of the second base transceiver station to the base transceiver stations connected to the mobile station;
at each of the base transceiver stations connected to the mobile station, e) determining whether the identification included in the notification received from the mobile station is identical to an identification assigned thereto; f) when the identification included in the notification is identical to the identification of its own, determining whether a radio channel used to communicate with the mobile station has sufficient Communication quality; g) when the radio channel has sufficient communication quality, decreasing the first reference SIR by a predetermined decrease amount; h) when the radio channel hag insufficient communication quality, determining whether the base transceiver station is involved in the closed loop control controlling transmission power of the mobile station, based on the number of times the notification has been received from the mobile station for a predetermined time period; i) when the base transceiver station is involved in the closed loop control, increasing the first reference SIR by a predetermined increase amount; and j) when the base transceiver station is not involved in the closed loop control, causing the first reference SIR not to be changed,
According to a fifth aspect of the present invention, in a mobile communications system composed of a network controller, a plurality of base transceiver stations connected to the network controller, and a plurality of mobile stations, a method for controlling a reference SIR (Signal-to-Interference power Ratio) which is used as a reference of a closed loop control by a base transceiver station to control transmission power of a mobile station connected to a plurality of base transceiver stations, includes the steps of:
at the network controller, a) receiving a frame of data received by each of the base transceiver stations from the mobile station; b) selecting a frame of data having best condition among a plurality of frames of data received by the base transceiver stations; c) counting the number of times a frame of data having best condition has not been selected for a predetermined time period to produce a non-selection count for each of the base transceiver stations; d) selecting a first base transceiver station having a non-selection count which is not smaller than a predetermined count threshold; and e) sending a non-selection notification to the first base transceiver station;
at each of the base transceiver stations connected to the mobile station, f) determining whether the non-selection notification is received from the network controller; g) determining whether a radio channel used to communicate with the mobile station has sufficient communication quality; h) when the radio channel has sufficient communication quality, decreasing the reference SIR by a predetermined decrease amount; i) when the radio channel has insufficient communication quality, determining whether the base transceiver station is involved in the closed loop control controlling transmission power of the mobile station, depending on whether the non-selection notification has been received from the network controller: j) when the base transceiver station is involved in the closed loop control, increasing the first reference SIR by a predetermined increase amount; and k) when the base transceiver station is not involved in the closed loop control, causing the first reference SIR not to be changed.
According to a sixth aspect of the present invention, in a mobile communications system composed of a network controller, a plurality of base transceiver stations connected to the network controller, and a plurality of mobile stations, a method for controlling a reference SIR (Signal-to-Interference power Ratio) which is used as a reference of a closed loop control by a base transceiver station to control transmission power of a mobile station, includes the steps of:
at the network controller, a) inquiring the reference SIR from each of the plurality of base transceiver stations; b) selecting a minimum reference SIR among a plurality of reference SIRS received from the bass transceiver stations; c) sending the minimum reference SIR to the bass transceiver stations;
at each of the bass transceiver stations receiving the minimum reference SIR from the network controller, d) replacing a current reference SIR with the minimum reference SIR received from the network controller.